In the oil and gas exploration and production industry, subsurface hydrocarbon-bearing formations are accessed from surface by drilling bores, which bores are subsequently lined with metal tubing known as casing or liner. In instances where the formation pressure exceeds the hydrostatic pressure produced by the column of fluid in the well bore, one or more valves must be provided in the well bore to prevent uncontrolled escape of hydrocarbons from the well. In contrast, in other instances, it may be desirable to protect the formation from hydrostatic pressure to prevent fluid flowing into, and damaging, the formation, and in such cases the column of fluid in the well bore must be isolated from the formation.
A wide range of valves have been proposed and used in well bores to cope with the situations outlined above and to provide for, for example, the safe deployment of tools and devices into the well bore.